Oscillators are used to generate frequencies for applications varying from relatively unsophisticated applications for wristwatches and the like, to such extremely sophisticated applications as timing systems for space navigational systems. Most commonly, quartz crystal resonators are used in oscillators, although certain highly accurate frequency standards can be configured using an atomic reference source, such as cesium or rubidium. However, the output frequency of a crystal oscillator varies with temperature and so these oscillators must operate in a temperature controlled environment in order to achieve high frequency stability. A common technique used to control the ambient temperature of a crystal oscillator is to put the crystal in a temperature controlled oven.
One approach that has been used to achieve higher frequency stability than the oven controlled crystal oscillator is the double oven controlled crystal oscillator (DOCXO). DOCXOs are similar in design to oven controlled crystal oscillators, with an additional outer oven wrapped around an internal oven controlled crystal oscillator. The outer oven means that DOCXOs have an additional layer of insulation over which the power input must balance the heat dissipation, resulting in another drop in maximum ambient temperature at which the oven will operate. Most DOCXOs use higher temperature set points, which increases aging, and compensate for this by using 3rd or 5th overtone crystals. This can cause a DOCXO to be bigger and require more power to operate.
Oven controlled crystal oscillators (OCXO) that are both stable and accurate, are highly desirable for use in many applications. A typical oven controlled crystal oscillator (OCXO) consists of a crystal based oscillator, a temperature control system, and support circuitry surrounded by a layer of thermal insulation enclosed in a sealed metal can. Inside the oven, the crystal is kept at a fixed temperature between 70 degrees Centigrade to 90 degrees Centigrade, based on the turnover point of the crystal where the frequency vs. temperature response is nominally flat. For example, an ovenized quartz crystal oscillator has means for controlling the temperature of a quartz crystal to an accuracy of less than 800 millidegrees centigrade. Currently, for example, stable and accurate OCXOs are sought for use in the design of base stations for cellular, PCS (personal communication system), and wireless local loop (WLL) systems that connect subscribers to a public switched telephone network. OCXOs can be used successfully, for instance, in the transmit and receive functions or in the clocks of CDMA (Code Division Multiple Access) base stations. The time error of a CDMA base station must be held to within 8.4 microseconds. This requirement forces an accuracy on the base station clock of a few milliHz for a 10 MHz signal which is difficult with an OXCO unless it is complex, relatively large and expensive.
In an OCXO, the crystal and associated components, the latter of which might also be sensitive to temperature, are enclosed in an oven that is kept at a stable temperature. Most temperature control systems are based on a thermistor/eror amplifier/semiconductor heater design. In order to maintain a constant temperature within the oven, there must be a balance of power input to the oven with heat flowing out of the oven. The temperature is kept constant by adjusting the amount of power supplied to the oven whenever the ambient temperature in the oven begins to change. The oven temperature selected is one at which the slope of the frequency vs. temperature curve for the crystal is zero. Thus, the oven minimizes the degree to which the frequency of the oscillator will vary with variations in temperature.
The realization of an OCXO typically requires: (1) a quartz crystal resonator as a primary reference element; (2) associated circuitry for frequency generation or synthesis; (3) a frequency tuning element or elements; (4) a thermal control system for the oven; and (5) an output buffer amplifier so that the signal output of the OCXO can be utilized.
More particularly, a typical oven controlled crystal oscillator (OCXO) consists of a crystal based oscillator and temperature control system and support circuitry surrounded by a layer of thermal insulation enclosed in a sealed metal can. Inside, the crystal is typically kept at a fixed temperature between 70 degrees centigrade and 90 degrees centigrade, based on the turnover point of the crystal where the frequency vs. temperature response is nominally flat.
Ovenized oscillators heat the temperature sensitive portions of the oscillator which is isolated from the ambient to a uniform temperature to obtain a more stable output. Ovenized oscillators contain a heater, a temperature sensor and circuitry to control the heater. The temperature control circuitry holds the crystal and critical circuitry at a precise, constant temperature. The best controllers are proportional, providing a steady heating current which changes with the ambient temperature to hold the oven at a precise set-point, usually about 10 degrees above the highest expected ambient temperature. Temperature induced frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The crystal for the oven is selected to have a “turning-point” at or near the oven temperature further reducing the sensitivity to temperature. The combination of the high oven gain with operation near turning point yields temperature stabilities as good as 1.0 ppb (one part per billion) over a temperature range that would cause the crystal to change by 10 ppm (one part per million). Highly polished, high-Q crystals which often have significant activity dips may be designed with no activity dips near the operating temperature providing better stability and phase noise than crystals designed for wide temperature ranges. Ovenized oscillators allow the use of SC-cut crystals which offer superior characteristics but which are impractical for ordinary TCXOs because of their steep frequency drop at cooler temperatures. Unfortunately SC cut crystals are much more expensive to produce than AT cut crystals typically used in TCXO's. Oven oscillators have a higher power consumption than temperature compensated oscillators. Oven oscillators require a few minutes to warm-up and the power consumption is typically one or two watts at room temperature.
In order to achieve an OCXO with a desired accuracy and stability, the precision of the reference element has always been of great significance. Unfortunately, the requirement for a precise reference element has limited the yield of crystal production and has kept the cost of creating OCXOs high. This is because the precision of the frequency of a crystal is affected by a great number of factors in the manufacturing process, such as the thickness of the cut of the crystal wafer, the angle of the cut, and imperfections or scratches on the crystal. The oscillator circuitry sensitivity to the frequency of the reference element likewise has contributed to manufacturing obstacles to large-scale and cost-effective OCXO production. Typical frequency-tuning components, such as inductors, capacitors and varactor diodes, are sensitive to environmental conditions, such as temperature, and repeatability and tolerance drift of these components over time must be taken into account in a typical OCXO design. The thermal control system for the oven has to be capable of achieving very accurate temperature settings uniquely adjusted for the characteristics of the particular reference element used.
When greater frequency stability is desired than can be provided by an OXCO other frequency stabilization approaches are used. One approach that has been used to achieve even higher frequency stability than the OCXO is the double oven controlled crystal oscillator (DOCXO). DOCXOs are similar in design to the OCXO, with an additional outer oven wrapped around the internal OCXO. That is, a temperature controlled oven contained within another temperature controlled oven is utilized. The outer oven means that DOCXOs have an additional layer of insulation over which the power input must balance the heat dissipation, resulting in another drop in maximum ambient temperature at which the oven will operate. Most DOCXOs use higher temperature set points, which increases aging, and compensate for this by using 3rd or 5th overtone crystals. This can cause a DOCXO to be bigger and require more power to operate.
More particularly, in a double oven configuration two insulated enclosures are utilized, one being placed inside the other with a proportionally controlled heater assembly for each. In order to maintain temperature control of the assembly under varying ambient conditions, it is necessary to maintain a differential of about 10 degrees Celsius between the highest ambient temperature to be experienced and the set point of the outer oven. Another 10 degrees Celsius differential is then required between the outer oven and the inner oven. The total heat rise above ambient between the crystal and the outside of the outer oven is more than 20 degrees Celsius. If the ambient temperature is high, the inner oven may need to operate at temperatures of around 110 degrees Celsius. There are several significant disadvantages of operating an oscillator at this high of a temperature. First, the reliability of electronic components decreases with temperature. The mean time between failures (MTBF) or operating lifetime of the circuit assembly is reduced as the temperature increases. In order to improve the failure rate, higher grade components must be used or more time spent screening components. Second, aging of the crystal resonator is accelerated. Crystals age more rapidly at higher temperatures. As the crystal ages, its frequency shifts causing frequency stability to rapidly degrade.
In addition, while present DOCXOs provide a relatively stable frequency output, as technology has advanced the need for oscillators having greater and greater stability has grown. Thus, there is a need in the art for ovenized crystal oscillators that provide even greater frequency stability than typically available in the prior art which is typically less than 0.1 part ppb.